Investigating inclusions and mechanical properties of 1060 aluminum by salt fluxes refining

Abstract

In the casting process of 1060 industrial pure aluminum, the inclusions in the aluminum melt significantly affect the product quality. In this study, the influence of refining temperature and the composition of salt fluxes on the purification effect and mechanical properties of aluminum melt was investigated. The results indicate that lower refining temperatures and modified salt fluxes can effectively enhance the cleanliness of the aluminum melt. As the refining temperature increases, the large inclusions gradually increase. The addition of 16wt.% Na3AlF6 can dissolve and break up Al2O3 inclusions, facilitating the separation of the aluminum melt and aluminum slag. The addition of 16wt.% Na3AlF6 and 2wt.% CaCO3 to the basic salt fluxes enables gas refinement, thereby further improving the cleanliness of the aluminum melt. Under the refining condition of 37wt.% NaCl-47wt.% KCl-16wt.% Na3AlF3-2wt.% CaCO3 at 740 °C, better cleanliness and mechanical properties were obtained. The cleanliness and yield strength are approximately 99.99928% and 71.46 MPa, respectively. This work can offer valuable reference and theoretical insights for future research.

1. Introduction

1060 industrial pure aluminum has many advantages, such as easy processing, good corrosion resistance, excellent thermal conductivity, and low density [1]. In the refrigeration industry, due to its low cost, 1060 industrial pure aluminum is usually used as a substitute for copper in producing important components such as evaporators, condensers, capillaries, and dry filters. However, it is challenging to eliminate inclusions from the aluminum melt during the purification process, which negatively affects the quality of aluminum products. In 1060 industrial pure aluminum, inclusions disrupt the matrix's continuity and exhibit notable differences in properties compared to the matrix. Non-metallic inclusions serve as typical sources of fatigue cracks. The larger the inclusion size, the lower the fatigue strength [2, 3]. The inclusions in the matrix tend to cause stress concentrations at the sharp corners. They are common sources of material fractures and corrosion [4, 5].

Gao et al. [6] analyzed the "hard spots" in Al-Si-Cu alloy and found that the primary compositions of these "hard spots" are Al2O3 and trace amounts of Al4C3. The hardness of these spots ranged from 818.1 HV to 1,425.7 HV, which was over 14 times that of the aluminum alloy matrix. The formation of Al4C3 is caused by the introduction of carbon in the aluminum production process, such as the direct contact between graphite electrodes and molten salts in the electrolytic aluminum process, and the reaction between different organic compounds and melts during the processing [7, 8]. He et al. [9] investigated the impact of CO2 generated through the anodic effect on the aluminum melt during the electrolysis process. They discovered that CO2 reacted with the aluminum melt to form α-Al2O3 at high temperatures. Many studies have confirmed that over 99% of the inclusions in aluminum melts are Al2O3 inclusions, including various types such as α-Al2O3, γ-Al2O3, and η-Al2O3. These inclusions can exhibit fragmented, granular, membranous, or plate-like shapes, with a wide size range from 1 nm to 1,000 μm [10, 11].

The preparation of high-quality 1060 aluminum ingots primarily depends on the purification level of the aluminum melt. Researchers have conducted extensive studies to efficiently remove inclusions from the melt [12-16]. Aluminum melt purification methods can be categorized into adsorption, non-adsorption, and composite purification. Among these methods, the salt fluxes method, as a kind of adsorption method, has become widely adopted in industrial production due to its advantages, such as ease operation, low cost, and high efficiency [17, 18]. It can effectively remove water from the solvent thoroughly, homogenize the components, and facilitate purification. Majidi et al. [18] investigated the impact of melting temperatures and adding salt fluxes on the refining process of A380 and A319 aluminum alloys. It is determined that the NaCl-KCl-Na2SiF6-CaF2 flux is effective in removing inclusions from aluminum melt at 740 °C. Shi et al. [19] determined the optimal composition of fluoride in salt fluxes. They studied the interfacial tension between aluminum alloys and different fluoride salts added at 740 °C. The results indicated that adding KF, NaF, Na3AlF6, MgF2, and AlF3 reduced the interfacial tension between t...